This invention relates generally to acoustic wave sensors such as hydrophones as used in calibrating lithotripters.
Lithotripters are medical shock wave devices which are used to disintegrate kidney and gall bladder stones. In an extracorporeal lithotripsy system, a shock wave transducer or spark discharge shock wave source generates shock waves which are focused and transmitted to a patient through a suitable fluid coupling medium. By positioning the stones in a patient at the focal point of the shock waves, the shock waves are transmitted to and disintegrate the stones.
The lithotripters must be properly calibrated prior to use. This is typically done by transmitting the focused shock waves in a fluid container and moving an acoustic sensor or hydrophone in the container to locate the true focal point and to measure the strength of the focused acoustic wave. A conventional hydrophone uses electrodes on a thin membrane to measure the shock waves. However, quite often the thin membrane will develop a dent in the active region after only a few shocks. This will adversely affect the response rise time and reproducibilty characteristics of the hydrophone. In order to prolong the life of such hydrophones, the thin membrane has been sandwiched between two layers of glycerine as a protective shield. However, this structure drastically changes the shape of the measured waveform, especially the negative (rarefactional) pressure region.
Another embodiment of conventional hydrophones is the reflector style design which basically consists of a ceramic or polymeric active element backed by a high acoustic impedance. The hydrophones built by the ceramic crystal in this manner have yielded high sensitivities coupled with acceptable rise times. The life span of such hydrophones is considerably improved over the membrane style hydrophones. However, this design will not detect negative pressures, and difficulties are associated in attempting to obtain a small, well defined active area.